The invention relates to electronic devices, and, more particularly, to equalization in multi-tone receivers and corresponding circuitry and methods.
Digital subscriber line (DSL) technologies provide potentially large bandwidth (e.g., up to 20 MHz for subscribers close to the central office) for digital communication over existing telephone subscriber lines (the copper plant). The subscriber lines can provide this bandwidth despite their original design for only voice band (0–4 kHz) analog communication. In particular, ADSL (asymmetric DSL) adapts to the characteristics of the subscriber line by using a discrete multi-tone (DMT) line code with the number of bits per tone (sub-carrier) adjusted to channel conditions. The bits of a codeword are allocated among the sub-carriers for modulation to form an ADSL symbol for transmission. FIG. 2 illustrates the use of the Fast Fourier transform in the system having, for example, 256 tones with each tone treated as a QAM channel (except dc tone 0) and so the kth tone encoding corresponds with a complex number X(k); 0≦k≦255. Extending to 512 tones by conjugate symmetry allows the 512-point IFFT to yield real samples x(n), 0≦n≦511, of the transformed block (symbol); and a DAC converts these samples into a segment of the transmitted waveform x(t).
The non-ideal impulse response of the transmission channel leads to interference of successive symbols (blocks), so each symbol is extended with a (discardable) cyclic prefix to allow for simplified equalization when the channel memory is less than or equal to the cyclic prefix length. For example, with a cyclic prefix of length 32 and symbol size 512 there are 544 samples transmitted; namely, x(480), x(481), . . . , x(510), x(511), x(0), x(1) . . . , x(479), x(480), . . . , x(510), x(511). Because the cyclic prefix contains duplicative information, shorter cyclic prefixes increase the overall transmission bit rate. Hence, various methods to shorten the effective channel impulse response with a time domain equalizer (TEQ) and thus maximize transmission bit rate have been proposed. For example, Arslan et al, Equalization for Discrete Multitone Transceivers to Maximize Bit Rate, 49 IEEE Tr. Sig. Proc. 1123 (2001) design a TEQ by minimizing the sum of the intersymbol interference power plus noise power for a given signal power. Nevertheless, these methods do not guarantee a TEQ that will be close to optimal in terms of maximizing the overall data rate and minimizing computational intensity.
In another approach, van Acker et al, Per Tone Equalization for DMT-Based Systems, 49 IEEE Tr. Comm. 109 (2001) translate the TEQ into frequency domain equalizers (FEQs), one FEQ for each tone; this allows optimization within each tone separately. But for systems with a large number of tones, such as ADSL with 256 tones, this becomes computationally burdensome.